A thin film transistor liquid crystal display device mainly comprises an array substrate, a color filter substrate, and a liquid crystal layer sandwiched between the array substrate and the color filter substrate. The array substrate comprises gate lines, data lines and a plurality of pixel regions by intersecting the gate lines and the data lines with each other. Within each pixel region, there are formed a thin film transistor and a pixel electrode. The pixel electrode is connected to a drain electrode of the thin film transistor through a via hole formed in a passivation layer covering the thin film transistor. A color filter layer formed of color filters of red, green and blue (R, G, B) and a black matrix are formed on the color filter substrate. The black matrix is disposed to correspond to the thin film transistor to prevent light leakage.
In a practical process, there is often a deviation when the array substrate and the color filter substrate are bonded with each other, and thus it is necessary to increase a width of the black matrix in order to avoid light leakage caused by such deviation. However, increase of the width of the black matrix will reduce an aperture ratio of the pixel region and degrade the display quality. In order to solve the above problem, a Color filter On Array (COA) technology in which the color filter layer and the black matrix are directly formed on the array substrate emerges. In the COA technology, the color filter layer and the black matrix are formed on the array substrate together with the thin film transistor, which not only can improve the aperture ratio and brightness of the display device, but also can avoid various problems caused by forming the color filter layer and the thin film transistor on different substrates.
However, in the COA technology in which the black matrix is formed on the array substrate, although the black matrix shields the thin film transistor and the metal wires (e.g., the gate lines and the data lines), the drain electrode provided at the via hole of the passivation layer cannot be shielded by the black matrix for the drain electrode provided at the via hole of the passivation layer needs to be connected to the pixel electrode. In this case, the drain electrode at the via hole will reflect the ambient light, resulting in reduction of contrast ratio and color purity of the display device, so that the display effect of the display device is seriously affected.